International Journal of Aquaculture, 2024, Vol.14, No.3, 154-164 http://www.aquapublisher.com/index.php/ija 159 administration of salmon or carp pituitary extract (PE) as an exogenous source of gonadotropins has been used to induce vitellogenesis in female European eels (Anguilla anguilla), which is crucial for reproductive success and subsequent growth (Kottmann et al., 2021). Furthermore, the use of Bacillus amyloliquefaciens (GB-9) and Yarrowia lipolytica lipase2 (YLL2) as dietary supplements has been shown to significantly improve growth performance, digestive enzyme activity, and innate immunity in Japanese eels (Anguilla japonica) (Zheng et al., 2019). These findings suggest that hormonal and supplementary interventions can be effective strategies for promoting growth in eel aquaculture. 5.3 Genetic factors influencing growth Genetic factors also play a significant role in the growth and development of eels. The expression of specific genes related to growth and development can influence the overall growth performance of eels. For example, the mRNA transcript profiles of growth-related genes such as growth hormone (gh), insulin-like growth factors (igf1, igf2a, igf2b), and thyroid hormone receptors (thrαb, thrβa, thrβb) have been shown to vary during embryogenesis in European eels (Anguilla anguilla) (Kottmann et al., 2021). Additionally, the expression of androgen receptors (arα, arβ) and follicle-stimulating hormone receptor (fshr) has been linked to the modulation of primary ovarian growth and vitellogenic development in Japanese eels (Anguilla japonica) (Lee and Lou, 2019). Understanding these genetic factors can provide insights into the mechanisms underlying growth and development in eels, thereby informing breeding and management practices to enhance growth performance. A combination of nutritional interventions, growth hormones and supplements, and genetic factors can significantly enhance the growth performance of eels in aquaculture. By leveraging these approaches, it is possible to improve the efficiency and sustainability of eel farming practices. 6 Integration of Genomic and Developmental Approaches 6.1 Combined strategies for improvement The integration of genomic and developmental approaches has shown significant promise in enhancing reproductive success and growth in eel species. One effective strategy involves the use of phytoestrogens, such as soy isoflavones, to manipulate sex differentiation. In Japanese eels (Anguilla japonica), genistein has been found to induce feminization more efficiently than daidzein, leading to a higher ratio of females, which are commercially more valuable due to their higher body weight (Inaba et al., 2022). This approach leverages the molecular pathways responsible for sex differentiation, as evidenced by the up-regulation of female-specific genes and down-regulation of male-specific genes in the gonads 6.2 Technological advances Technological advancements have played a crucial role in the application of genomic and developmental approaches to eel aquaculture. The development of specific recombinant gonadotropins, such as European eel (Anguilla anguilla) recombinant gonadotropins (aarGths), has enabled precise control over the reproductive processes (Kottmann et al., 2021). These recombinant hormones, produced in Chinese hamster ovarian cells, have been used to induce spermatogenesis and spermiation in captive male eels. The use of a combination of recombinant follicle-stimulating hormone (aarFsh) and luteinizing hormone (aarLh) has been particularly effective, with the best results achieved through a constant dose of aarFsh and an increasing dose of aarLh (Peñaranda et al., 2018). This technological innovation has significantly improved sperm quality and reproductive success in European eels. 6.3 Case studies of integrated approaches Several case studies highlight the successful integration of genomic and developmental approaches in eel aquaculture. In one study, Japanese eels were fed a diet treated with estradiol-17β or genistein for 45 days (Inaba et al., 2022). The study found that adding soy isoflavones to the feed can significantly increase the proportion of female eels, with the effect being more pronounced when genistein is added (Inaba et al., 2022). The experiments demonstrated that genistein could induce feminization by upregulating female-specific genes (such as foxl2a and cyp19a1) and downregulating male-specific genes (such as amh and gsdf) (Figure 3). Additionally, the study
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